Data Transformation

Data Transformation with Dasel v3

Activate this skill when:

• Modifying values in config files (YAML, TOML, JSON, XML, HCL, INI)
• Converting between data formats (JSON to YAML, TOML to JSON, etc.)
• Batch-updating array elements (prices, versions, flags)
• Appending, removing, or reshaping data structures
• Merging objects or constructing new structures from existing data

Safety Rules

NEVER redirect output to the same input file. This truncates the file before dasel reads it, resulting in data loss:

# WRONG — destroys input
cat config.yaml | dasel -i yaml --root 'port = 9090' > config.yaml

# CORRECT — write to temp, then rename
cat config.yaml | dasel -i yaml --root 'port = 9090' > config_tmp.yaml && mv config_tmp.yaml config.yaml

Always verify before overwriting. Preview the transformation output first, then redirect:

# Preview
cat config.yaml | dasel -i yaml --root 'server.port = 9090'
# Apply
cat config.yaml | dasel -i yaml --root 'server.port = 9090' > config_tmp.yaml && mv config_tmp.yaml config.yaml

Core Modification Syntax

In dasel v3, put and delete subcommands do not exist. All modifications use assignment expressions with --root to output the full document.

Set a Value

# Output full document with one field changed
cat config.yaml | dasel -i yaml --root 'server.port = 9090'

# Numeric, boolean, and string assignments
echo '{"count": 1}' | dasel -i json --root 'count = 42'
echo '{"enabled": false}' | dasel -i json --root 'enabled = true'
echo '{"name": "old"}' | dasel -i json --root 'name = "new"'

Set Nested Values

cat config.yaml | dasel -i yaml --root 'database.connection.host = "db.example.com"'
cat config.yaml | dasel -i yaml --root 'database.connection.port = 5432'

In-Place Modification (Safe Pattern)

# Single field update
cat config.yaml | dasel -i yaml --root 'server.port = 9090' > config_tmp.yaml && mv config_tmp.yaml config.yaml

# Multiple fields — chain with semicolons
cat config.yaml | dasel -i yaml --root 'server.port = 9090; server.host = "0.0.0.0"' > config_tmp.yaml && mv config_tmp.yaml config.yaml

Format Conversion

Pipe through dasel with different input/output format flags:

# JSON to YAML
cat data.json | dasel -i json -o yaml > data.yaml

# YAML to TOML
cat config.yaml | dasel -i yaml -o toml > config.toml

# JSON to XML
cat data.json | dasel -i json -o xml > data.xml

# TOML to JSON
cat config.toml | dasel -i toml -o json > config.json

# CSV to JSON
cat data.csv | dasel -i csv -o json > data.json

Array Operations

Append to Array

# Add element to end of array
echo '[1,2,3]' | dasel -i json --root '[$this..., 4]'
# Output: [1, 2, 3, 4]

# Append object to array
cat data.json | dasel -i json --root 'items = [$root.items..., {"name": "new", "value": 42}]'

Batch Update with each

# Multiply all prices by 1.1 (10% increase)
cat data.json | dasel -i json --root 'items.each(price = price * 1.1)'

# Set a flag on all elements
cat data.json | dasel -i json --root 'users.each(active = true)'

# Increment all values
echo '[1,2,3]' | dasel -i json --root 'each($this = $this + 1)'

Filter Then Transform

# Get only active users, then extract names
cat data.json | dasel -i json 'users.filter(active == true).map(name)'

Object Operations

Merge via Spread

# Add new key to existing object
cat base.json | dasel -i json --root '{ $root..., "newKey": "value" }'

# Merge two objects
cat base.json | dasel -i json --root '{ $root..., "extra": true, "version": 2 }'

Field Deletion via Reconstruction

Since v3 has no delete command, remove fields by constructing a new object with only the desired keys:

# Keep only "name" and "email", drop everything else
echo '{"name":"a","email":"b","password":"c"}' | dasel -i json --root '{ name, email }'

Field Rename via Reconstruction

echo '{"old_name": "value"}' | dasel -i json --root '{ "new_name": old_name }'

Multi-Step Transformations

Use variable assignment and semicolons for complex operations:

# Store intermediate result in variable, then transform
cat data.json | dasel -i json '$active = users.filter(active == true); $active.map(name)'

# Multiple variables
cat data.json | dasel -i json '$a = items.filter(price > 100); $b = $a.map(name); $b'

Conditional Transformation

# Set value based on condition
cat data.json | dasel -i json --root 'if(count > 5) { status = "many" } else { status = "few" }'

Transformation Patterns

For detailed per-use-case transformation patterns (config updates, batch processing, format migration, data reshaping, error handling), see Transformation Patterns.

References

• Dasel v3 Documentation (fetched 2026-02-19)
• Dasel Spread Operator (fetched 2026-02-19)
• Dasel each() Function (fetched 2026-02-19)
• Dasel filter() Function (fetched 2026-02-19)
• Dasel CHANGELOG (fetched 2026-02-19)